docs: def check_dimensionless_vertical_coordinate(self, ds):
          '''
          Check the validity of dimensionless coordinates under CF
  
          CF §4.3.2 The units attribute is not required for dimensionless
          coordinates.
  
          The standard_name attribute associates a coordinate with its definition
          from Appendix D, Dimensionless Vertical Coordinates. The definition
          provides a mapping between the dimensionless coordinate values and
          dimensional values that can positively and uniquely indicate the
          location of the data.
  
          A new attribute, formula_terms, is used to associate terms in the
          definitions with variables in a netCDF file.  To maintain backwards
          compatibility with COARDS the use of these attributes is not required,
          but is strongly recommended.
  
          :param netCDF4.Dataset ds: An open netCDF dataset
          :rtype: list
          :return: List of results
          '''
          ret_val = []
  
          z_variables = cfutil.get_z_variables(ds)
          deprecated_units = [
              'level',
              'layer',
              'sigma_level'
          ]
          for name in z_variables:
              variable = ds.variables[name]
              standard_name = getattr(variable, 'standard_name', None)
              units = getattr(variable, 'units', None)
              formula_terms = getattr(variable, 'formula_terms', None)
              # Skip the variable if it's dimensional
              if (formula_terms is None and
                  standard_name not in dimless_vertical_coordinates):
                  continue
  
              is_not_deprecated = TestCtx(BaseCheck.LOW, self.section_titles["4.3"])
  
              is_not_deprecated.assert_true(units not in deprecated_units,
                                            "§4.3.2: units are deprecated by CF in variable {}: {}"
                                            "".format(name, units))
              ret_val.append(is_not_deprecated.to_result())
              ret_val.append(self._check_formula_terms(ds, name))
  
          return ret_val